1. Field of the Invention
The present invention relates to a defect correcting apparatus for a solid state image sensor.
2. Related Background Art
There are already known solid state image sensors utilizing semiconductor such as charge-coupled device or CCD.
A CCD is composed of a silicon semiconductor substrate bearing an SiO.sub.2 layer on a face thereof, on which electrodes are formed with a regular pitch. An optical image is projected from the side of the electrodes or from the opposite side, thereby accumulating electric charges in the portions of the semiconductor positioned under the electrodes, and the thus accumulated charges are transferred in succession and read out by clock pulses supplied to the electrodes.
Such solid state image sensor utilizing semiconductor tends to generate local defects in the crystal structure due to the difficulty in the preparation of uniform semiconductor crystal over a desired area, and there may result an abnormally high dark current because a charge is easily generated in the position of such defect by a thermal phenomenon. For this reason, in projecting an optical image and reading the resulting signals as explained above, noises are generated corresponding to the positions of such high dark current.
Thus, as shown in FIG. 1, a noise N higher than the white level may become present in the image signal So and become conspicuous in the reproduced image.
An already proposed method for eliminating such noise utilizes a memory. In this method, the memory remembers the crystal defects in the semiconductor substrate and controls the output signal from the solid state image sensor so as to eliminate the noises.
Such a memory stores information on the presence or absence of crystal defects, usually the information given for each pixel.
Consequently, a CCD having N.sub.H and N.sub.V pixels respectively in the horizontal and vertical detections requires a memory capacity of N.sub.H.N.sub.V bits. In order to obtain an image equivalent to the usual television image, the value of N.sub.H is in a range of 300 to 500, and that of N.sub.V is in a range of 200 to 300. The storage of crystal defects in the above-explained method requires a large memory capacity, and the solid state image sensing apparatus cannot be obtained inexpensively since the defect correcting device of this method requires an expensive memory circuit.
FIG. 2 is a block diagram of a conventional image sensing apparatus, provided with a CCD 1, a sample and hold circuit 2, an amplifier 3, a clock pulse generator 4, a flaw memory 5 storing the information on the crystal defects in advance, a gate circuit 6, and accumulated image signal SO.
FIG. 3 is a wave form chart showing the relationship between the sample and hold circuit and the gate circuit for a crystal defect, wherein (a) indicates an output signal to the sample and hold circuit 2, and (b) indicates an output signal of the gate circuit 6.
In such apparatus, the positions of the pixels having crystal defects in the CCD 1 are stored in advance in the memory circuit 5, and the image signals are read, as shown in FIG. 3(a), by clock signals from the clock pulse generator 4. A signal from a normal pixel is periodically reset, in the sample and hold circuit 2, by a signal supplied from the clock pulse generator 4 through the gate circuit 6, but the resetting is not effected in a position of a defective pixel to replace such defective signal with the retained signal of the immediately preceding pixel.
However, in case a striped or mosaic filter of for example red, green and blue is employed in such conventional image sensing apparatus, the luminance signal can be formed in this method without difficultly because the signal of the defective pixel has a strong relation to that of the immediately preceding pixel but the chroma signal cannot be formed by this method because the preceding pixel is of a different color. In this manner the correction of a noise in the chorma signal has resulted in the formation of a false signal.
For reducing the memory capacity there has also been proposed to store the positions of crystal defects in encoded form, instead of storing the presence or absence of defect in each pixel. The position of a crystal defect can be encoded by the X- and Y-coordinates of the defect on the plane of the semiconductor substrate. The position of a pixel in the horizontal scanning direction can be represented by 9 bits in case the number N.sub.H of pixels in the direction is of the order of 500. Also the position of a pixel in the vertical scanning direction can be represented by 8 bits in case the number N.sub.V of pixels in the direction is of the order of 300 in an interlace scanning method, plus 1 bit for identifying whether the defect is present in an odd field or an even field.
Consequently the position of a defect can be represented by 18 bits, including X and Y coordinates and field identification. Thus, if a CCD can tolerate for example 20 defects at maximum, the total memory capacity can be reduced to about 400 bits.
FIG. 4 shows an example of a conventional defect compensating apparatus employing such memory, wherein the CCD is driven by an interline transfer method as shown in FIG. 5.
As shown in FIG. 5, the CCD is provided with a plurality of pixels arranged in vertical direction, and each column of pixels is associated with a vertical shift register 8 for transferring charges. The charges transferred by the vertical shift register 8 are then transferred by a horizontal shift register 9 pixel by pixel and are read through a terminal 10.
There are shown an image taking pulse P.sub.I, a transfer pulse P.sub.V supplied to the register 8, and a read-out pulse P.sub.H supplied to the horizontal register 9.
Then referring to FIG. 4, the image of an object 16 is projected through an optical system 17 onto a CCD 15, and an output signal obtained at the terminal 10 is supplied, through a sample and hold circuit 18, to an output terminal 19. The sampling state of the sample and hold circuit 18 is controlled by sampling pulses P.sub.S synchronized with the read-out pulses P.sub.H and controlled by the output of the memory.
There is provided a memory device (defect memory circuit) 25 composed for example of a read-only memory and storing the positions of defects in encoded form. An address counter 35 for the CCD is composed of a horizontal counter 35H for counting the horizontal position and a vertical counter 35V for counting the vertical direction. The horizontal counter 35H receives the read-out pulses P.sub.H, and a reset terminal thereof receives a horizontal synchronization signal HD as a reset signal.
Similarly the vertical counter 35V receives the transfer pulses P.sub.V, and a vertical synchronization signal VD as a reset signal.
A position signal S.sub.L obtained in the counter 35 is supplied, together with a field signal S.sub.F indicating an odd field or an even field, to an identity circuit 36, which also receives an output S.sub.M of the defect memory circuit 25 and supplies an identity signal S.sub.Q when the output S.sub.M of the defect memory coincides with the position signal S.sub.L and the field signal S.sub.F, together with the sampling pulse P.sub.S, to a gate circuit 37. In this state the output S.sub.Q is shifted to "1" to interrupt the output P.sub.SO of the gate circuit, thereby interrupting the function of the sample and hold circuit 18 and causing the sample and hold circuit to retain the state of an immediately preceding pixel.
Consequently a defect noise present in the image signal in this period is eliminated by the function of the sample and hold circuit 18 and is compensated by the signal of an immediately preceding pixel.
The identity output signal S.sub.Q is also supplied to the address counter 38 to release the position of a succeeding defect. A reset terminal 38a thereof receives a signal supplied at an interval of the frame.
However such defect compensating apparatus, in which the signal of a defective pixel is replaced by the signal of an immediately preceding pixel, may generate false signals in case the image of the object has no correlation in the horizontal direction, thus resulting in a significant deterioration in the image quality.